Method of and arrangement for stepwise tuning of electric circuits



June 28, 1938. F. scHwARzER m5221183 METHOD OF AND ARRANGEMENT FOR'STEPWISE TUNING OF ELECTRIC CIRCUITS Filed April 8, 1936 2 Sheets-Sheet-l mjl m Il o Il D ull ull b Il V v s v; S

l'vENToR @wg mez L, BY Ml ,f /f/ i June 28, 1938. 2,122,183

METHOD oF AND'ARRANGEMENT FOR' sTEPwIsE TUNING 0F ELECTRICv CIRCUITS FL scHwARzERf Filed April 8 1956 2 sheets-sheet 2 connec feo INVENToR 15d/m TORNEYS Patented June 2s, 1938 PATENT OFFICE METHOD F ANDI ARRANGEMENT FOR STEPWISE TUNING OF ELECTRIC CIR- CUITS Fritz Schwarzer, Berlin- Schoneberg, Germany Application April 8,A 1936, Serial No. 73,380

In Germany Claims.

My invention relates to a method of and arrangement for stepwise tuning of electric circuits, such as may be employed for instance in adjusting the frequency of electric circuits in the radio and kindred arts.

It is already known to provide devices especially for tuning high frequency communication circuits with Step-wise adjustable tuning elements which operate by two or more selective procedures, and by which stepwise rst coarse steps and then within such coarse steps ne steps are adjusted. With such arrangements preferably different fixed capacities are selectively connected into or out of circuit. In such a case, as is well-known, considerable difliculties are encountered in utilizing the same equal fine frequency step arrangement for all coarse steps 4and to obtain within allcoarse steps the same uniform division of the fine frequency steps. Therefore, in such cases additional, separate expedients had te be used heretofore in order to obtain 'approximately equal frequency steps when the same fine step arrangement is used for all coarse steps` `In some of the well-known arrangements of this character the practical construction still encounters considerable diiculties. For instance, the original dimensioning of the different step. values is very difiicult. Since in such arrangements indirectly also those condenser electrodes act as series capacities which are not connected in circuit at the time, these indirect capacity effects mustbe taken into consideration and in-` cluded into the calculation, not only when the system is first calculated, but, in dimensioning in the assembly each condenser electrode to a definite value, the total indirectly acting capacity of the remaining electrodes also varies, so that in progressively dimensioning the different capacity step values of the system the already dimensioned steps change in their effect upon the system so that correct and permanent values can'- Y not be fixed.

The invention refers in particular to such step tages are avoided. According to the invention a combination of parallel and series connected capacities are used whereby the coarse steps are produced by assembling the totalcapacity of each coarse step from at least two parallel connected capacities. Each ne step is then produced by individual series connection of the selected condenser of a fine step condenser group with only one of the constituent condensers of the selected coarse step. With such an arrangement the relative values of the constituent elements of each coarse step are dimensioned so that on one hand the desired coarse capacity step is produced,

and on the other hand the additional selective Y a solution by which the above-mentioned disadvan- April 11, y1935 (Cl. Z50-40) series connection of a similar ne step result in the required line variation of each coarse frequency step at the same ne frequencyinterval.

My invention is illustrated in the accompanying drawings in which- Fig. 1 represents diagrammatically the method of arranging the different capacities for coarse and nne step adjustment, and

Figs. 2 and 3 represent a constructive form of such an arrangement of which Fig. 2 represents o a sectional elevation onl line 2 2 in Fig. 3, and Fig. 3 a sectional elevation on line 3-3 in Fig, 2. First, the method of connection will be explained with reference to the circuit diagram Fig. 1. The tunable circuit, which may for instance be a portion of a circuit arrangement for a thermionic tube T, consists of an inductance L'and of a number of condensers, of which in the present case always three simultaneously partake in the tuning. These condensers are taken from three groups which together constitute the coarse and fine step tuning system, and which in this figure is arranged as a decimal system. The condensers Ca to Ck together with the condensers of group Cm to 01k constitute the coarse steps (which may for instance be 100 kilocycles apart) and the condensers C1 to C10 represent the fine steps (which may be for instance kilocycles apart).

The connecting means for effecting the coarse and fine condenser connection which is required for the different steps is represented by the sliding elements a: and y assumed in this diagram as sliding contacts. The leads u and w represent the common low potential and ground connections of one side of the condensers of the coarse group Ca to Ckand of the condensers of the fine group C1 to C10,- and the lead 'urepresentsthe common connection of the low potential side of all condensers Cia to Cik which side may be connected in series with any selected fine step condenser. The fine steps, it will be noted, are thus connected for each coarse step in seriesrwith-only a portion of the total coarse step capacity. If now the total coarse step capacity of a given step is distributed over the two condensers of the two coarse step groups, appertaining to that step, ata certain ratio which can easily be calculated, the additional selective series connection of the fine step condensers brings about within all coarse steps the same fine frequency intervals. In order to prove this assertion it is only necessary to consider the two extreme cases. If for instance in' a given coarse step the constituent capacity portion Ca is made to equal zero, the condenser C must include the entire capacity necessary for the frequency appertaining to that step. If m such a case the fine steps are added in order to make fine adjustments these fine steps are thrown in series with the total coarse capacity, and considerable variations in the uniformity of the steps will occur when this is tried in the different coarse steps. If, on the other hand, the capacity C13 is made to equal zero, the entire required coarse capacity must be embodied in Ca. The additional series connection of fine steps, with C ia equal Zero, would in this case not produce any' The manner in which these capacitiesV can be calculated in practice is as follows:-

According to the invention, the total capacity CR of the circuit is composed for the coarse steps of a parallel capacity Cp, a series capacity Cs and a ne step capacity CF." vIf these capacities are combined, we have the resulting value The capacity of the circuit wires and other circuit elements is for the moment disregarded, since it can be easily correctly added to Vthe parallel capacity Cp. 1

If we further assume that the inductance ofthe circuit is constant, the frequency ofthe oscillatory circuit then becomes dependent only upon the capacity of the circuit (Thomsons formula). The frequency, .according to this formula, -Would then be. Y

whereby Fine f frequency steps C4, C5, C0, C7, C3, C9, C10.

Athe values Ca and C15 (of C1 and C10) are assumed, all other unknown values in thc ten equations can quency step we have the equation according to Equation 3:

g for the first coarse and second ne step:

. E CalLcaa-cz Thus ten equations are derived for each coarse step with the unknown values Ca, C10, C1, C2, C3,

If for practical reasons and so forth.

be calculated, which when combined according to the invention produce ten equally spaced frequencies, constituting 4the small steps, for instance 10 kc. each.

Thus after the capacity values of the ne steps are determined, the values of CP and Cs can be determined for each coarse step likewise on the basis of Equation 3. For instance for the second coarse step and rst fine step we have the equation K 2 C1bC1 fbl12- b C15-lq1 For the second i i coarse step, l l v tenth ine2 :Cb Clb'clo andsoforth for f0102 0111+ C10 the remaining step coarse steps.

Thus for each coarse step, the desired' frequency range can be determined (for vinstance kc.) Since, however, each coarse step is divided into ten fine steps according to the above equations, it follows that a change by one fine capacity step will produce at least substantially, if not exactly, the desired fine frequency step (for instance'lO kc.)

For calculating a set of coarse and ne capacities in designing a step by step tuning device according to the present invention, one would proceed as follows in substance in accordance with the preceding mathematical deductions.

First, the desired stepyalues for the coarse andV ne capacities are entered into a table in relation to one another. 'It may here be assumed that the desired intervals ofthe fine step group amount to about 9 kc. each. The diieren'ces between the coarse steps 'are not exactly 90 kc. but slightly differ from one another for the purpose of Yequalizing inequalities in the distribution of the wave lengths of the several transmitters over the given frequency range. Thus, Vthe following Table I results, in which thetop row represents fblO fal, fa2, fa3, fa4, fa5, fa, fa7, faS, faQ, falO Coarse fbl, fb2, Y Y frequency steps UWe can then set up the following equations. For instance for the rst coarse and rst fine frethe fine steps `1-10 and the rows A-K represent the coarse steps in. kc.

TABLEI Frequencies in kc.

0 9 3 7v t 0 5 4 3 2 1 F 901 970 079 Y 933 997 1000 1015 1024 1033 1042 K `1054 1003 1072 v 1031 1090 1099 1103 1117 1120 1135 1 1140 1155 1104 1173 1132 1191 1200 1209 1213 1227 H 1233 1247 1250 1205 1274 1233 1292 1301 1310 1319 o 132s 1037 1340 1355 1304 1373 1332 1391 1400 1409 11 1419 1423 1437 1440 1455 1404 1473 1432 1491 1500 11 1510 1519 152s 1537 1540 1555 1504 1573 1532 Y 1591 D 1000 1009 1013 1027 1030 1045 1054 1003 1072 1031 o 1091 1700 1709 1713 1727 1730 1745 1754 1703 1772 B ,1731 1790 1799 1 1303 `1317 l c1320 1335 1344V 1053 -1352 A Then the capacities 'appertaining to each frequency are calculated. The smallest tuning capacity for the given circuit was assumed to be 135 cm. with which is coordinated the highest frequency to which the given circuit can be tuned, namely, 1862 kc. (A-1, Table I). These cal- If thiscondition is entered in Equation I, we have Il. CK"

Assuming that the capacities of a coarse step are known, the line steps are culated -values are then entered into the following HI F GS(CR- GP) Table II. (iq- (CRHQ) TABLE II Capacities in cm.

506 81 497. 45 488. 34 479. 48 470. 86 462. 47 454. 3l 446. 37 438. 62 431. O7 K 421. 32 414. 21 407. 30 400. 53 393. 94 387. 52 381. 23 375. 14 369. 16 363. 32 I 356. 39 350. 86 345. 45 340. 17 335. 0l 329. 97 325. 04 320. 21 315. 49 310. 89 H 305. 39 300. 99 296. 70 292. 49 288. 37 284. 33 280. 40 276. 52 272. 74 269. 04 G 265. 40 261.84 258. 34 254. 92 251. 58 248. 28 245.06 241. 9 238.8 235. 7 F 232. 45 229. 53 226. 66 223. 85 221. 09 218. 38 215. 72 213. 1 210. 49 208. 02 E 205. 27 202. 85 200. 47 198. 13 195. 83 193. 57 191. 34 189. 16 187. 02 184. 90 D 182. 83 180. 79 178. 78 176. 81 174. 88 172. 96 171.08 169. 24 167. 42 165. 63 C 163. 69 161. 95 160. 26 158. 58 156. 93 155. 31 153. 71 152. 14 150. 59 149. 06 B 147. 56 146. 08 144. 62 143. 18 141. 77 140. 37 139.00 137. 65 136. 31 135. 00 A For the further calculation there should be deducted froln the foregoing calculated values a constant value representing the capacity dis- If two fine steps are known, the values Gs and GP are to be calculated for all coarse steps as followstributed over the whole circuit, which value in C G 1 G5147a the present case is assumed 80 cm. Thus, the P GS.;- 1 9a actual tuning capacity values result which are GSF, entered in the following Table III. Cb- GP+ GS+ Fb TABLE III Capacities in cm.

0 9 s 7 9 5 4 a 2 l F 425. s1 417. 45 40s. 34 399. 4s 39o. 89 582. 47 574. 91 s55. 37 358. 62 351. o7 1; 341. 52 394. 21 327. 90 52o. 53 313. 94 907. 52 901. 25 295. 14 289. 15 285. 92 l 270. 59 270.55 265. 45 250. 17 255. 01 249. 97 245. 04 24o. 21 2.25. 49 239. 99 H 225. 99 220. 99 215. 70 212. 49 20s. 37 294. 35 209. 39 195. 5 2 192. 74 189. 04 G 185. 40 181. 84 178. 34 174. 92 171. 58 168. 28 165. 06 161. 90 158. 80 155V 76 F 152. 45 149. 55 145. 69 143. 85 141.09 las. 58 155. 72 155. 10 150. 54 129. 02 n 125. 27 122. 85 120. 47 118.15 115. s3 119. 57 111. 109.16 107. 02 104. 90 D 192. 59 100.79 98. 79 99. s1 94. 9s 92. 95 91. 89. 24 87. 42 85. 99 o s3. 69 s1. 95 s0. 25 7s. 5s 75. 95 75. 91 75. 72. 14 70. 59 59. 05 B 07. 55 60. 08 54. 02 59. 1s 91. 77 99. 37 59. 57. 05 55. 31 55. 09 A For the further calculation, the following additional designations are introduced:

If these equations are solved according to their unknown, we have The capacity of the capacity combination C provided with the index of the coarse step (A-K) and the ne step- (1-0) The parallel capacity GP with the index of the coarse step (A-K) The series capacity Gs with the index of the coarse step (A-K) The capacity of the ne step F With the index of the ne step (1-0) The fundamental equation on which the further calculation is based is- GSF I. CR- G P om For practical reasons the condition was required that the values of largest individual capacities should be approximately all of the same order GFK: GSK: F o

Thus, all equations for the division of the capacities are determined.

The numerical evaluation was made as follows. First, after the coarse step K, the ne steps yF1 and F0 were calculated. From the values obtained were then determined the parallel-and the series capacities for the coarse step H (GPH, GSH) and thereafter definitely the line steps Fl-Fu. From the ne steps F9 and. F2 the coarse steps A-J are calculated. In order to determine the deviations. on the step K, the capacity GFK was 'increased a corresponding amount.` Thus, the following values were obtained:-

GP Gs F In order to determine the deviations, all capacities are combined according to the system described hereinbefore, and the resulting values are entered in the following Table IV.

The idea involved in such 'an arrangement as Vshown in Fig. 1 may be embodied in a practical apparatus in the manner shown in Figs. 2 and 3.

Referring to these figures, the three diiTerent groups of condensers above-mentioned are arranged separately on three circular carriers Z, m, n, of suitable dielectric material, for instance ceramic material of well-known type, and so that the high potential electrodes of condensers C6 to Ck are located on the side plate Z, facing plate m, the high potential electrodes of the condensers C111 to C111 are arranged on the side on plate m facing plate Z, and the high potential electrodes of condensers C1 to C10 are mounted on the front side of platen. These individual electrodes may consist of suitable metal, such as silver, and may be attached to their respective carrier plates by spraying or other suitable means. Their exact TABLE IV Capacities in cm.

428. 12 417. 496 407. 53 398. -18 389. 40 381. 138 373. 336 365. 952 358. 968 352. 379 K 341. 709 334. 210 326. 969 320. 033 313. 372 306. 979 300. 847 294. 876 289.160 283. 570 J 276. 387 270. 86 265. 451 260. 17 255.011 249. 98 245. 042 240. 234 235. 491 230. 912 H 225.175 220.990 216.853 212. 746 208.676 204.645 200, 650 196.680 192.750 188.878 G 185. 086 181. 840 178. 571 175. 296 172. 011 168. 724 165. 427 162. 115 158. 795 155. 490 11 152. 114 149. 530 146. 914 144. 267 141. 587 138. 878 136. 136 133. 353 130. 540 127. 704 E 124. 949 122. 85 120.723 118. 554 116. 340 114. 085 111. 781 109. 424 107. 020 104. 574 D 102. 499 100. 79 99. 030 97. 226 95. 371 93. 468 91. 511 89. 492 87. 42 85. 293 o The differences 2 between the required values and the actually resulting Values are entered in the following Table V.

areas may be iinally adjusted by grinding or scraping away surplus portions, a method of adjustment,V well-known in the manufacture of TABLE V Capacities in cm.

+1.31 +o. 046 0.81 1.30 1. 46 1. 33 0. 97 0.42 +0.35 +1.31 K +0.39 :1:0 0.33 0.5 0.57 0. 54 0.38 0.26 :1:0 +0.35 J +0 i0 :1:0 io :to +0. 01 :1:9 +0.02 :1:0 +0.02 H 0. 21 i0 +0. 15 +0. 26 +0. 31 +0. 32 +0` 26 +0. 16 +0. 01 0. 16 G 0.30 :1:0 +0. 23 +0.38 +0. 43 +0. 44 +0. 37 +0.21 i0 0. 27 F 0.34 +0 +0. 25 +0.42 +0. 5o +0. 50 +0.42 +0. 25 i0 0.32 E 0.32 i0 +0. 25 +0. 42 +o. 51 +0. 52 +0. 44 +0. 26 :80 0.33 D 0. 83 :t0 +0. 25 +0. 42 +0.49 +0. 51 +9. 43 +0.25 i0 0.34 o 0. 32 :t0 +0. 22 +0. 39 +0.48 +0. 48 +0. 41 +o. 24 :60 0. 32 B 0. 29 i0 +0. 22 +0. 37 +0. 45 +0. 46 +0.38 +0. 22 :1:0 0.32 A

The largest capacity deviation exists at the ceramic condensers. The three carrier plates are Value K6, amounting to 1.46 cm. This would correspond With a frequency diierence of 1.56 kc. The greatest frequency diierence exists, on the other hand, in the step A, since here the greatest relative capacity difference appears. At A the error is +0.46 cm. or +2.95 kc.

In the following Table VI are entered the tuning deviations in kc. Errors below 0.5 kc. are denoted by 0, since they may be neglected for all practical purposes.

mounted in parallel to one another at suitable distances apart and are fixed in a frame 0. To plates Z, m, and n on the side where the high potential electrodes are located are xed respectively the conducting contact stars in, q, T, the shape of which is shown with respect to the star r in Fig. 2. The arms sof each of these stars constitute yielding tongues, as shown in Fig. 2, which protrude severally'over the sector-shaped high potential condenser electrodes, as shown in both TABLE VI Frequency deviations i711 icc.

0 0 o +0. 75 +0.75 +o. 75 +0. 5 0 o 0. 5 J

0 o 0 1.5 0.75 0. 75 0.5 0 0 0 G +0.75 o 0.5 1. 0 1. 25 1. 25 1. 0 0` 5 0 +0.75 F +1. 0 0 0. 75 1. 25 1.50 1. 50 1.25 0. 75 0 +1.25 E +1.25 o 1.00 1. 50 2. 00 2.00 1. 75 1. 00 0 +1. 5o D +1.50 o 1. 00 2.00 2.25 2.50 2.00 1. 25 o +1.75 o +1.75 0 1. 25 2.00 2.50 2.75 2.25 1. 50 0 +2. 00 B +1. 75 0 1. 25 2.00 2.75 3.00 2.50 1. 50 o +2.25 A

ISO

figures, and which are provided at these overlapping portions each with a silver contact point t. As may be further noted from Fig. 2, the contact stars p, q, 1 are mounted eccentrically with respect to the circular carriers Z, m, n on which latter the condenser electrode elds are arranged in eccentric sector fashion with different areas. The low potential counterelectrodes for the electrode sectors on carriers l, m, n constitute closed metallic rings u, u, w, indicated in Fig. 1 with correspondingly similar letters, which in that figure are meant to include the low potential condenser plates as well as the interconnecting leads. The switching elements .r and y in Fig. 1 correspond in Figs. 2 and 3 to the rotatable arms n: and y, together with the pressing balls e1, z2 respectively, of which ball .22, is made of insulating material for purely constructive reasons. Arm is mounted insulated on the rotatable shaft a1 which carries at one end a hand knob d'2. This arm constitutes one of the terminals of the entire vcondenser arrangement which is connected, as

shown in Fig. 1, to lead x which is in turn connected to the high potential end of the nductance L. In Fig. 3 this connection is merely indicated. by an arrowed line with the legend To 91. l The contact arm 'J is mounted on the hollow shaft h1 which is vcarried on shaft a1, and shaft b1 is provided with a hand knob be. Of course, in place of these hand knobs any other suitable driving means for these two shafts may be used in accordance with the mechanism in which the condenser system may be mounted.

If now for instance arm is rotated by operating its hand knob a1, the contact ball 21 carried by that arm will press oppositely located tongues of stars p and q, between which arm :r is mounted, outwardly as soon as it encounters these tongues and thereby press these tongues with their respective contact tips t against their oppositely located condenser electrode sectors. In this manner two appertaining condensers of the twocoarse groups Ca to C1; and. Cm to C11; are connected in parallel, the same as if in Fig. 1 the slide :I: had been moved for instance into the position shown in that figure. If now by rotating arm y into the desired position, its contact ball 22 will press tip t of the particular tongue of contact star r which it encounters against the oppositely disposed line step condenser sector of the fine step group C1 to C10, and thereby connect a particular ne step condenser in series with the previously selected parallel connected coarse step condenser of group Cia to Cik. For instance, as shown in Fig. 1 the slide y may connect the fine step C4 in series with the constituent condenser C1@ of the total coarse capacity step Ce, Cie.

For simplicity of illustration the carrier plate n shown in Fig. 2 in front elevation is provided with only eight sectors C1 to Cs instead of using the decimal system shown in Fig. 1. This is, of course, only a matter of design.

The essential advantage of the arrangement shown for instance in Figs. 2 and 3 is that within the capacity groups no separate connections are necessary. Aside from the modifications shown and described hereinabove numerous additional modifications of this arrangement may be made within the scope of the present invention, involving a mixed series and parallel connection of capacities as tuning elements in oscillatory circuits or the like, for obtaining uniform or approximately uniform frequency steps. For instance, according to the invention it is also possible to arrange a third group of condensers which constitute a further subdivision of the fine group C1 to C10, and which permit within that group stepwise adjustments from kilocycle to kilocycle.

I claim: l. Means for stepwise tuning an electric circuit by capacity variation at substantially uniform frequency intervals, comprising a group ofl individual coarse step capacities and at least.

one group of selected individual ner step capacities constituting the substeps for the coarse capacities, each of said coarse capacities being divided into two parallel-connected individual constituent capacities at such a ratio with respect to the fine step capacities, that each selected ner step capacity, when connected in series with one of the constituent capacities of any selected coarse step, produces the same ne step frequency intervals, whereby the same group of finer step capacities can be used as substeps for each coarse step capacity to produce substantially the same nner stepI frequency intervals for all coarser capacity steps.

2. Means for stepwise tuning an electric circuit by capacity variation at uniform frequency intervals, comprising a group of individualcoarse step capacities and -at least one group of individual ner step capacities constituting the substeps for the coarse capacities, each of said coarse capacities being -divided with respect to the finer step capacities at such a ratio into two parallelconnected individual constituent capacities that each selected finer step capacity, when connected in series with one of the constituent capacities of any selected coarse step, produces the same fine step frequency intervals, a contact spring for each constituent capacity of each coarse step, and means for simultaneously actuating the contact springs of the constituent capacities of the selected coarse step to connect said capacities in parallel into the circuit, a contact spring for each finer step capacity and means for actuating the spring of the selected finer step capacity to connect it in series with one of the constituent capacities of the selected coarse step, whereby the same group of ner step capacities can be used as substeps for each coarse step capacity to produce substantially the same ner step frequency intervals for all coarse capacity steps.

3. Means for stepwise tuning an electric circuit by capacity variation at uniform'frequency intervals, comprising a group of individual coarse step capacities and at least one group of individual fine step capacities constituting the substeps for the coarse capacities, each of said coarse capacities being divided with respect to the finer step capacities at such a ratio into two parallelconnected individual constituent capacities that each selected finer step capacity, when connected in series with one of the constituent capacities of any selected coarse step, produces the same fine step frequency intervals, a disc ofy dielectric material having one of the electrodes of one set of constituent capacities of all coarse steps mounted on one side in sector fashion and carrying on the opposite side an annulus of conductive material representing the other electrode in common to all first-named sector electrodes, a second similarly constructed disc of dielectric material having the electrodes of the other set of constituent capacities mounted on it in similar sector and annulus` fashion, said discs beingv spaced apa-rt and fixed in parallel `to one another so that the sectors of the constituent capacities of each coarse step are located opposite one another, contact means rotatable between said discs for connecting the constituent sectors of the selected coarse step simultaneously in parallel into the circuit, `a third disc of dielectric material constructed similar to the previously mentioned. discs and having the electrodes of the fine step capacities mounted on it in similar sector and annulus fashion, and contact means rotatable over the sectorized portion of said disc for connecting the selected ne ca- Y pacity in series with one of the constituent capacities of the selected coarse step, whereby the same group of fine step capacities can be used as substeps for each coarse step capacity to produce substantially the same ne step frequency intervals for all coarse capacity steps.

4. Means for stepwise tuning an electric circuit by capacity variation at uniform frequency intervals, comprising a group of individual coarse step capacities and at least one group of individual fine step capacities constituting the substeps for the coarse capacities, each of said coarse capacities being divided with respect to the fine step capacities at such a ratio into two parallelconnected individual constituent capacities that each selected flnerstep capacity, when connected in series with one of the constituent capacities of any selected coarse step, produces the same fine step frequency intervals, a disc of dielectric material having one of the electrodes of one set of constituent capacities of all coarse steps mounted on one side in sector fashion and carrying in the opposite side an annulus of conductive material representing the other electrode in common to all rst-named sector electrodes, a second similarly constructed disc of dielectric material having the electrodes of the other set of constituent capacities mounted on it in similar sector and annulus fashion, said discs being spaced apart and fixed in parallel to one another so that the sectors of the constituent capacities of each coarse step are located opposite one another, a star-shaped contact spring xed on each disc and having each of its arms extending over one of the constituent electrode sectors on said disc, and means rotatable betweenl said discs for simultaneously actuating the contact arms for the constituent capacities of the selected coarse step, to connect said capacities simultaneously in parallel into the circuit, a third disc of dielectric material constructed similar to the previously mentioned discs and having the electrodes of the ne step capacities mounted on it in similar sector and annulus fashion, a star-shaped contact spring xed on said disc and having eachV of its arms extending over one ofthe sectors of the ne step capacities, and means rotatable over said arms for actuating the contact arm of the selected fine step to connect the selected fine step capacity in series with one of the constituentcapacities of the selected coarse step, whereby the same group of ne step capacities can be used as substeps for each coarse step capacity to produce substantially the same ne step frequency intervals for all coarsev capacity steps.

5. Means for stepwise tuning an electric circuit by means of a plurality of xed capacities serving as coarse tuning steps, each capacity being divided at a suitable ratio into a plurality of parallel-connected portions, and a plurality of capacities serving as fine tuning steps, said tuning means comprising means for connecting the two parallel portions of a selected coarse step into the circuit and means for connecting into the circuit for each selected coarse step one of the fine step capacities in series with one of the constituent portions of the selected coarse step,

to produce for each tuning step a definite total Y capacity resulting from a plurality of capacities in mixed series-parallel connection, the relative values of the parallel connected portions of each coarse step being dimensioned with respect to the fine step capacities so that the same nner step capacities produce in all coarse steps substantially the same ne step frequency intervals.

FRITZ SCHWARZER. 

